The sensitivity to electrostatic discharge of energetic compositions, including solid propellant, gas generating, and pyrotechnic compositions, is well known. Numerous sources of electrical discharge have been cited as possible causes of catastrophic explosions or premature ignition of rocket motors containing solid propellants. External sources include natural lightning, electromagnetic pulses, high power microwave energy, and the like. In addition, static electricity charges are normally present at the interfaces between the various phases in the propellant, insulation, liner, and other parts of the rocket motor. Charging of surfaces may occur by surface-to-surface contact (triboelectric contact) and by the cracking or separation of the solid phase, as in fractoelectrification.
Sudden discharge of this electrostatic energy may result in an explosion of materials or generate sufficient heat to ignite the solid propellant. Such catastrophic events have the potential for causing harm to people and property.
One manufacturing operation which has been implicated as a cause of catastrophic discharge and premature propellant ignition is the core pulling operation, i.e., removal of the core molds from the solid propellant grain after the grain is cast. Other manufacturing operations have the potential for causing rapid electrostatic discharge. Such events may also occur during storage, transportation, and deployment of materials or rocket motors.
The safety properties of any energetic material, whether it is a neat component or an energetic formulation such as a propellant or PBX (Plastic Bonded Explosive), are of primary concern when handling such materials. Friction, impact, and electrostatic sensitivity are measured for each energetic material and compared to standards, typically pentaerythtritol tetranitrate (PETN) or cyclotrimethylenetrinitramine (RDX). The military has determined a pass/fail value for each test; should a material fail any of these tests, limitations on its handling are imposed. The test to determine electrostatic sensitivity involves discharging a stored charge through a needle to a metal crucible containing the energetic material and monitoring for adverse reactions (fire, explosion, etc.) The pass/fail value for this test is 5000 volts (0.25 joules) and mimics the maximum charge a human can discharge after static build up. Coating energetics with conducting polymers or using a conductive polymer as part of a binder system could dramatically reduce electrostatic sensitivity.
Composite solid propellants have a very complex microstructure consisting of a dense pack of particles embedded in a polymeric binder matrix. The particles typically comprise fuel, oxidizers, combustion control agents, and the like. The particles may have a wide variety of sizes, shapes, and electrical properties. Electrostatic charges typically build up on the binder-filler interfaces, on the grain surface, as well as at the interfaces between other components of the propellant, e.g. at the interface between conductive particles such as aluminum powder and a nonconductive or less-conductive binder.
Certain propellant compositions have a greater conductivity than other compositions. For example, a propellant having a polar polymer may contain dissociated ionic species available for charge transport and would have relatively high conductivity. Such ionic species may be present from ammonium perchlorate dissolved in the polar binder. Electrostatic charges are readily dissipated and catastrophic discharge is unlikely with this type of propellant binder system.
One approach to reduce the electrostatic sensitivity of a particular formulation is to blend in a small amount of graphite (˜1%). However, this is often not successful, and addition of such materials detracts from the performance as energetic components are sacrificed to include them. For example, BTATZ, which has acceptable friction and impact sensitivity, does not pass the electrostatic sensitivity test (fires at 0.15 joules) as a neat component or when blended with poly(ethyl acrylate) as a binder. Addition of graphite to the poly(ethyl acrylate)/BTATZ formulation shows no noticeable improvement. Another approach involves using containers coated with fatty amines, but this only contributes to safe storage of materials and to prevent static adherence to the container. Other approaches involve slurrying the energetic components with binder components, such as poly lauryl methacrylate and hydroxy-terminated polybutadiene, to produce a coated explosive material (CXM), a free flowing powder free of static attraction, with decreased sensitivity to electrostatic initiation.
U.S. Pat. No. 3,765,334 issued Oct. 16, 1973 to Rentz et al. reports adding graphite to igniter compositions to prevent electrostatic charge build up. It is reported that at least 16 percent graphite is required to achieve adequate conductivity. Such amounts of graphite adversely affect performance of energetic materials.
U.S. Pat. No. 4,072,546 issued Feb. 7, 1978 to Winer and U.S. Pat. No. 4,696,705 issued Sep. 29, 1987 to Hamilton, teaches graphite fibers in solid propellant and gas generating compositions to provide structural reinforcement and burn rate control. However, it is known that even small amounts of graphite fibers markedly increase the processing viscosity of propellant compositions. Even slight increases in viscosity can detrimentally affect processing and propellant rheology.
In another propellant, the solid constituents are bound in a polybutadiene/acrylonitrile/acrylic acid terpolymer binder (PBAN). The binder polymer contains polar nitrile functional groups along its backbone. In this system, a quaternary benzyl alkyl ammonium chloride is added to the binder polymer during manufacturing. The polymer and the quaternary ammonium salt together provide a relatively high electrical conductivity.
Another commonly used binder system in solid rocket propellant compositions is hydroxy-terminated polybutadiene (HTPB). In contrast to the poly(ethylene glycol) (PEG) and PBAN binder systems, HTPB binders are nonpolar and have an intrinsic high insulation value. Thus, HTPB-based propellants are more susceptible, under certain circumstances, to high charge build-up with the potential for catastrophic electrostatic discharge.
Some pyrotechnic compositions are comprised of solid particles embedded in polymers and are susceptible to electrostatic discharge, as are solid propellants. Some pyrotechnic compositions are prepared without binders. The ingredients are either mixed dry or in an evaporative solvent. Dry mixing of pyrotechnic ingredients is particularly susceptible to electrostatic discharge. It is generally known that as air flows across a surface, charge buildup occurs. In dry mixing, there is a very large surface area, creating the potential for charge buildup and electrostatic discharge.
Low loading levels of conducting polymers (1–5% of the total weight) have been shown to effectively dissipate charge in coatings and textiles, leading to anti-static applications in microelectronics, fabrics, and carpeting. Fabrics coated with conducting polymers exhibit conductivities orders of magnitude higher than those seen in carbon-filled fabrics, likely due to improved continuity of the conducting portion of the fabric. Nanocomposites are formed by deposition of conducting polymers onto a wide variety of particle types, leading to bulk conductivities one to two orders of magnitude less than the corresponding pristine conducting polymer.
From the foregoing, it will be appreciated that there is a need in the art for an electrostatic charge dissipation system, which produces compositions having energetic particles with sufficient conductivity to reduce electrostatic discharge susceptibility, yet which are processible, retain energetic performance, and retain comparable ballistic, mechanical, and rheological properties.